Occupational epidemiology has its roots in classical medicine. However, it became a quantitative discipline only in the 20th century, through the pioneering work of individuals such as Case, Lloyd, and Selikoff and organizations such as the Division of Occupational Health of the U.S. Public Health Service. Studies of chemical dye workers, bituminous coal miners, smelting workers, and uranium miners have been especially important sources of innovations in methodology and in development of logical reasoning leading to acceptance of causal relationships of occupational exposures that lead to respiratory diseases and cancer. The cooperation of labor unions, such as those of steel and asbestos workers, has often been a crucial factor in providing essential data.